1. Field of the Invention
The present invention relates to an optical transceiver unit comprising an optical transmitter and an optical receiver housed integrally in a single casing.
2. Description of the Related Art
It is known of late that an optical communication system using optical fiber as a transmission line is widely employed for a variety of purposes ranging from a basic transmission line in public communication channel to a short-distance network such as LAN (local area network). In the optical communication system, an optical transmitter having a light emitting element and performing electro-optical conversion is provided on the transmission side, while an optical receiver having a light receiving element and performing opto-electric conversion is provided on the reception side. It is important particularly in a short-distance network to contrive a compact structure which incorporates both an optical transmitter and an optical receiver at low production cost in the optical transmission system.
For enabling bidirectional transmission, both an optical transmitter and an optical receiver need to be incorporated in a communication terminal apparatus and, in view of production cost, an advantage is attainable by mounting such transmitter and receiver on a single printed circuit board. However, the optical transmitter requires a great current for modulation of a light emitting element such as a laser diode, whereas the optical receiver deals with a feeble current obtained from the light receiving element and is therefore prone to be affected electromagnetically by the optical transmitter, so that an adequate electromagnetic shield means needs to be carried out between them.
Under such technical background mentioned above, it has been generally customary heretofore to adopt exemplary proper methods of: (a) disposing an optical transmitter and an optical receiver on the same or different printed circuit boards with a sufficient space kept therebetween; or (b) housing an optical transmitter and an optical receiver in individual shield cases separately. However, according to the first method (a), it is difficult to attain a high mounting density with a concomitant problem that the whole unit is rendered dimensionally greater. Meanwhile according to the second method (b), a rise of the production cost is unavoidable in addition to a dimensional increase of the whole unit.
Some exemplary optical transceiver units developed in an attempt to shield an optical receiver electromagnetically from an optical transmitter while averting a dimensional increase of the whole structure are disclosed in, for example, Japanese Patent Laid-open Nos. Sho 61 (1986)-88624, Hei 1 (1989)-16035 and Hei 1 (1989)-98299. In each of the conventional optical transceiver units described in the above, a light receiving element module and a receiving circuit are mounted on one surface of a printed circuit board where, e.g., two shield layers are laminated, while a light emitting element module and a transmitting circuit are mounted on the other surface of the printed circuit board.
In any of such known optical transceiver units, the shield layers for electromagnetically shielding the component elements of the receiving circuit inclusive of a preamplifier and so forth from the transmitting circuit are formed integrally with the printed circuit board, so that the whole unit can be down-sized to a certain extent. However, since the transmitting circuit is greater in circuit scale than the receiving circuit, the size of the printed circuit board is determined depending on the circuit scale of the transmitting circuit in the above-described related art which mounts the light receiving element module and the receiving circuit on one surface of the printed circuit board while mounting the light emitting element module and the transmitting circuit on the other surface thereof.
Consequently it follows that a sufficient margin space is left on the surface of the printed circuit board where the receiving circuit is to be disposed, and such a structure is undesirable in view of attaining a high mounting density. Further, since lead terminals on the printed circuit board project from its one surface, there exists another problem that the receiving circuit or the transmitting circuit mounted on the surface with the projecting lead terminals is harmfully affected by electromagnetic interference caused due to the electromagnetic waves emitted from the lead terminals.